Curing agent for fluorocarbon polymer coating compositions

ABSTRACT

A curing agent for fluorocarbon polymer coating compositions selected from the following group: ##STR1## where R is R 1  NH, R 1  is an aliphatic or cycloaliphatic hydrocarbon radical, R 2  is C or an aliphatic hydrocarbon group, R 3  is H or CH 3 , where x is 3 when R 2  is a hydrocarbon group and x is 4 when R 2  is C; the curing agents form ambient curing fluorocarbon polymer coating compositions; 
     These compositions are used to provide corrosion and abrasion resistant coatings for large structures in chemical plants, oil refineries, oil drilling platforms, and the interior of smoke stacks of large utility companies.

BACKGROUND OF THE INVENTION

This invention is related to a curing agent and in particular to acuring agent for a fluorocarbon polymer coating composition.

Fluorocarbon polymers are inert to strong acids such as sulfuric acid,nitric acid, hydrochloric acid and strong bases such as sodium hydroxideand are resistant to weathering and salt water corrosion and are toughand abrasion resistant. Coatings of these polymers would be useful inchemical plants and oil refineries to coat pipes, vessels and otherequipment, on off shore oil well platforms, on ships, and as protectivecoatings for the interior of smoke stacks of utility companies.Fluorocarbon polymer coatings would be particularly useful for metalsmoke stack interiors which are subjected to abrasion from fly ash andcorrosion by acids resulting from combustion products such as SO_(x) andNO_(x) and halogen ions. However, conventional fluorocarbon polymercoatings require curing at elevated temperatures which could not be usedon the aforementioned structures. An ambient curing fluorocarbon polymercoating composition is required.

The curing agent of this invention is used in fluorocarbon polymercoating composition and provides a composition that cures at ambienttemperatures.

SUMMARY OF THE INVENTION

A curing agent for fluorocarbon polymer coating compositions selectedfrom the following group: ##STR2## where R is R¹ NH₂, R¹ is an aliphaticor cycloaliphatic hydrocarbon radical, R² is C or a hydrocarbon, R³ is Hor CH₃ ; where x is 3 when R² is a hydrocarbon group and X is 4 when R²is C.

DETAILED DESCRIPTION OF THE INVENTION

Curing agent (1) is prepared by reacting 1 mole of dialkyl maleate with3 moles of an aliphatic or cycloaliphatic polyamine at about 100°-150°C. for about 1-6 hours. Alkanol formed in the reaction is removed.Preferred reaction conditions are about 110°-130° C. for about 2-4hours. Preferably, the above constituents are in an alkanol solvent suchas methanol, propanol isopropanol and the like.

Typical dialkyl maleates that can be used to form the curing agent aredimethyl maleate, diethyl maleate, dipropyl maleate, diisopropylmaleate, methyl ethyl maleate, methyl propyl maleate, ethyl propylmaleate, dibutyl maleate and the like. Dimethyl maleate is preferred.

Typical polyamines used to form the curing agent are isophorone diaminewhich is 3-aminomethyl-3,5,5-trimethylcyclohexylamine, hexamethylenediamine, ethylene diamine, 1,4-cyclohexane bis(methylamine),1,2-diaminopropane, propylene diamine, diethyl ether diamine,trimethylhexamethyl methylene diamine.

One preferred curing agent is the reaction product of isophorone diamineand dimethyl maleate and has the following structural formula: ##STR3##

Curing agent (2) is prepared through a Michael's reaction in which amultifunctional acrylate or methacrylate is reacted with a polyamine.The polyamine is heated to about 100°-150° C. and then themultifunctional acrylate or methacrylate is reacted with the polyaminefor a 1-6 hour period to form an amine terminated curing agent.

Typical multifunctional acrylates or methylacrylates that can be used toform curing agent (2) are trimethylol propane acrylate, trimethylolpropane methacrylate, pentaerythritol acrylate, pentaerythritolmethacrylate and the like. Any of the aforementioned polyamines can beused to form the curing agent (2).

The curing agents are used in fluorocarbon polymer coating compositionsor primer compositions. Generally, about 5-25% by weight, based on theweight of the binder of the composition, of curing agent is used.

One of the advantages of compositions that contain the curing agent isthat these compositions cure at ambient temperatures and baking is notrequired. Therefore, the compositions can be used on large structuressuch as chemical storage tanks, chemical reactors the interior of smokestacks and the like which could not be subjected to baking temperaturesusing conventional techniques.

Typical coating composition contains about 10-70% by weight binder and30-90% by weight of an organic solvent, in which the binder is

a fluorocarbon polymer of vinylidene fluoride and hexafluoropropyleneand has a weight average molecular weight of about 50,000-300,000 andcontains

a metallic oxide such as magnesium oxide which is an acid acceptor.

Usually the composition contains a reinforcing pigment such as titaniumdioxide or carbon black.

Molecular weight, as used herein, is determined by gel permeationchromatography using polymethylmethacrylate as a standard.

Preferably, the polymer used in the coating composition contains about50-70% by weight of vinylidene fluoride and 30-50% by weight ofhexafluoropropylene. The polymer can contain up to 40% by weight ofother monomers such as tetrafluoropropylene. One useful polymer containsabout 20-30% by weight of tetrafluoroethylene.

The metallic oxide which is an acid acceptor is used in the compositionto react with the hydrofluoric acid which is generated during the curingor crosslinking reaction. Typical metallic oxides are magnesium oxide,lead oxide, calcium oxide, lead hydrogen phosphate and a mixture ofcalcium oxide and magnesium oxide. Magnesium oxide is preferred.

Generally, the binder of a coating composition contains about 55-90% byweight, of the fluorocarbon polymer, 5-25% by weight of one of the aboveamine curing agents and 5-20% by weight of a metallic oxide which is anacid acceptor.

The coating composition also can contain dispersed fluorocarbon polymerssuch as polytetrafluoroethylene, fluorinated ethylene/propylenepolymers, polyvinyl fluoride, polyvinylidene fluoride, copolymer oftetrafluoroethylene/perfluoroalkoxy vinyl ether and the like. Thesedispersed fluorocarbon polymers are present in amounts of about 5-20% byweight, based on the weight of the binder of the composition.

Preferably, the coating composition contains a reinforcing agent such astitanium dioxide pigment usually in a pigment to binder weight ratio ofabout 20:100 to 200:100. Other inert pigments can be used such asbarytes, barium sulfate, fibrous calcium silicate and the like. Carbonblack, bone black or lamp black can also be used as a reinforcingpigment in a pigment to binder weight ratio of about 20:100 to 50:100.

Typical organic solvents that are used in the coating composition areacetone, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, propylacetate, butyl acetate, isobutyl acetate, methyl isobutyl ketone, methylamyl acetate, diisobutyl ketone, ethylene glycol monomethyl etheracetate, ethylene glycol monoethyl ether and mixtures of the above.These solvents are used to make the compositions and can be used toreduce the compositions to an application viscosity.

To decrease curing time and increase toughness of the resulting finishof the coating composition about 0.01-3% by weight based on the weightof the binder, of a bicyclic amidine can be added. One preferredbicyclyic amidine is 1,8-diaza-bicyclo(5,4,0)undecene-7.

Generally, the coating composition is sold in two components. Thesolvents, fluorocarbon polymer, pigments such as titanium dioxide andmetallic oxide acid acceptor are the first component and the aminecuring agent is the second component. The second component is blendedwith the first component by the user to form a coating composition. Thecomposition is then reduced with one of the aforementioned solvents toan application viscosity and then applied to a substrate.

The coating composition can be applied directly over a wide variety ofsubstrates and provide a fluorocarbon polymer coating. Typicalsubstrates are treated or primed steel, phosphatized steel, grit blastedsteel, galvanized steel, aluminum, copper, brass, cement andcementitious surfaces such as fire brick, mortar used for fire brick andthe like.

Generally, the coating composition is sprayed applied to the substrateand the solvent is allowed to flash off between coatings then theresulting coating composition is cured at ambient temperatures. Thecoating can be cured in about 4 to 48 hours or longer or can be heatedto 80° C. to 120° C. for 0.5 to 2.0 hours for rapid curing. Curedcoatings are about 75-1500 microns thick.

For some substrates such as untreated steel, a fluorocarbon primer isfirst applied and then while the primer is still wet the coatingcomposition is applied and dried at ambient temperatures or at the aboveelevated temperatures.

One useful primer contains the aforementioned fluorocarbon polymer, ametallic oxide acid acceptor such as magnesium oxide and anaminoalkyl-alkoxy silane such as amino-propyl trimethoxysilane oramino-propyl triethoxysilane.

The ambient curing characteristics of the coating composition providedby the novel amine curing agent of this invention allows for theapplication of coatings on large vessels and reactors in chemicalplants, and oil refineries, large structures and equipment and pipes,heat risers, i.e., pipes which are used to transport oil from theunderground well to the structure, off shore oil well platforms, and onthe interior of smoke stacks used by large utility companies. It ispractical to use compositions that cure at ambient temperature for theabove applications.

The following examples illustrate the invention. All parts andpercentages are on a weight basis unless otherwise indicated.

EXAMPLE 1

A curing agent was prepared by charging the following constituents intoa reaction vessel equipped with a stirrer, a heating mantel and acondenser:

    ______________________________________                                                       Parts by                                                                      Weight                                                         ______________________________________                                        Portion 1                                                                     Isophorone diamine                                                                             535.5                                                        Portion 2                                                                     Dimethyl maleate 144.0                                                        Portion 3                                                                     Isopropanol      516.0                                                        Total            1195.5                                                       ______________________________________                                    

Portion 1 was charged into the reaction vessel and heated to about 120°C. and then portion 2 was added at a uniform rate over a 4 hour periodwhile the temperature was held at 130° C. and methanol was removed fromthe vessel. After the 4 hour period heating was stopped and a sample wasremoved and tested in an infrared spectrophotometer which showed thereaction was complete. Then portion 3 was added to give a 55% solidssolution.

About 5 parts of the above curing agent was added to about 300 parts ofeach of the following coating compositions:

Coating Composition A

Fluorocarbon polymer solution containing 33% solids in ethyl acetate, ofa copolymer of 40% hexafluoropropylene and 60% vinylidine fluoridehaving a weight average molecular weight of about 100,000 and about 5%by weight magnesium oxide.

Coating Composition B

Fluorocarbon polymer solution identical to composition A except thecopolymer has a molecular weight of about 200,000.

Coating Composition C

Fluorocarbon polymer solution identical to composition A except thecomposition contains additionally about 25% by weight of finely dividedpolyvinylidene fluoride.

Each composition was diluted with methyl ethyl ketone to a sprayviscosity and sprayed onto aluminum and steel panels to form a coatinghaving a dry film thickness of about 1000 microns. After drying underambient temperature conditions for 7 days, each of the coatings isresistant to sulfuric acid, sodium hydroxide, steam and methyl ethylketone which indicates that the coatings were fully cured andcrosslinked.

EXAMPLE 2

The following amine curing agents were prepared:

    ______________________________________                                                       CURING AGENT                                                                  Parts by Weight                                                               D     E       F       G                                        ______________________________________                                        Isophorone diamine                                                                             510     --      562   --                                     Hexamethylene diamine                                                                          --      321     --    388                                    Trimethylol propane acrylate                                                                   296     296     --    --                                     Pentaerythritol acrylate                                                                       --      --      298   298                                    Isopropanol      659     505     703   562                                    Total            1465    1122    1563  1248                                   ______________________________________                                    

In the preparation of each of the above curing agents D-G, the amine wascharged into a reaction vessel and heated to 120°-140° C. and then theacrylate was slowly added at a uniform rate over a 4 hour period andthen the reaction mixture was cooled and isopropanol added.

Trimethylol propane acrylate and pentaerythritol acrylate were preparedby conventional techniques well known to the skilled in the art in whichan acrylate moiety was attached to trimethylol propane and withpentaerythritol.

Separate coating compositions were prepared with each of the curingagents D-G using the same constituents as in Example 1 for the coatingcomposition A except the above curing agent was substituted for thecuring agent of Example 1. In each case the resulting coatingcomposition was reduced to a spray viscosity with methyl ethyl ketone asin Example 1 and sprayed onto grit blasted steel panels allowing thecoating to flash dry between each application to provide a 1000 micronthick dry coating. After 7 days, the coatings were fully cured but weresofter and more elastic than the coating of Example 1. The coatings wereresistant to sulfuric acid, sodium hydroxide and methyl ethyl ketone.

We claim:
 1. A curing agent for fluorocarbon polymers consisting of##STR4## wherein R is R¹ NH₂ and R¹ is an aliphatic or cycloaliphatichydrocarbon radical.
 2. The curing agent of claim 1 in which R¹ is analiphatic hydrocarbon radical.
 3. The curing agent of claim 2 in whichR¹ is --CH₂ --₆.
 4. The curing agent of claim 1 in which R¹ is acycloaliphatic hydrocarbon radical.
 5. The curing agent of claim 1having the formula ##STR5##